The present invention relates to wavelength routers, particularly to a wavelength router for fiber optical networking and computer interconnects, and more particularly to a wavelength router based on a diffraction grating which utilizes only N wavelengths to interconnect N inputs to N outputs, and the grating may be combined with additional couplers or wavelength selective elements.
Wavelength division multiplexing (WDM) is becoming increasingly important as a means to increase the bandwidth available in fiber optic systems. These include telecommunications systems based on single-mode fiber, local area networks based on single strands of multimode fiber, and high performance computer interconnects based on parallel arrays of optical fiber. All these systems require wavelength multiplexers and routers to provide for independent transport of communications channels at different wavelengths. This has spawned a substantial industry in WDM components for telecommunications (currently several $100 Ms/year), which is expected to grow substantially within this market, and to extend beyond this market into local area networks based on the recent introduction of optical transport for gigabit ethernet.
All these systems can benefit substantially from a wavelength router, a device with N inputs and N outputs which routes light from a given input to an output which is determined by the wavelength of the light. The ideal device requires only N system wavelengths (λ). This is important because it minimizes system complexity, and minimizes the number of different laser transmitters which must be available to provision a system. The routing table for such a device may be as follows:
Output Ports:1234. . .NInput1λ 1λ 2λ 3λ 4 . . .λ N2λ Nλ 1λ 2λ 3 . . .λ N-13λ N-1λ Nλ 1λ 2 . . .λ N-24λ N-2λ N-1λ Nλ 1 . . .λ N-3. . .Nλ 2λ 3λ 4λ 5 . . .λ 1where λx indicates which wavelength x is used to connect a given input port to a given output. A key feature of the routing table is that it is fully non-blocking; that is, a connection between any input i and any output j can be established without disrupting or interfering with already existing connections.
A device called a waveguide grating router which provides this functionality is currently commercially available. It is a phased array operating in a high spectral order, and the ‘wrap-around’ (in which λN is directed back to port 1, etc.) is achieved by using multiple orders of the phased array. A similar approach could be achieved using a grating in high order. These devices must operate in high order to maintain a nearly constant dispersion across multiple orders. For example, a Littrow grating's dispersion (change in routed wavelength with output port position) is:
      Δλ          Δ      ⁢                          ⁢      x        =      Fp    ⁢                  cos        ⁢                                  ⁢        θ            d      
Where F is the working distance, θ is the output angle relative to the grating, d is the grating tooth pitch, and p is the spectral order. For the dispersion at fixed output position (θ) to be insensitive to order, p must be very large. Then ≈p±1 and the dispersion is almost constant.
The fundamental problem with operating in high spectral order is that the free spectral range (total usable wavelength range) of the device is limited to λo/p, where λo is the center of the operating wavelength range. For this reason, waveguide grating routers are only used for very dense WDM systems, in which wavelength channels are spaced very closely together. This is often undesirable, because it requires very accurate control of the operating wavelengths of all system components. If the wavelength channels are spaced far apart, only a few channels can be included in the system (which is undesirable).
A second problem with waveguide grating routers is that they are only available for single mode fiber optics. To achieve similar functionality for multimode fibers, one must use a conventional grating. However, it is difficult to obtain high-order gratings (echelles) that exhibit good diffraction efficiency at telecommunications wavelengths. As a result, grating-based wavelength routers typically either: 1) do not conserve wavelengths (they use many more wavelengths than N), 2) or they use more than N output ports, or 3) they require optoelectronic conversion. Each of these cases is discussed below.
Case 1. In general, a grating with N inputs, each supporting M wavelengths, generates M+N−1 output spots from a single diffraction order, where each spot is a unique output spatial position. To reduce the number of output spots to N (since there are only N output ports), one can employ more system wavelengths (2N−1). However, this increase in system wavelengths is undesirable for the reasons discussed above (increased system complexity and provisioning difficulty). An example of this approach is described in A. M. Hill et al., Photonics Technol. Lett. 8(4), 569 (1996), who uses 7 wavelengths for a 4×4 router (N=4). Another example is J. P. Laude et al., Proc. European Conf. Optical Commun. Vol. 3, pp. 87–90 (1997). “Very dense N×N wavelength routers based on a new diffraction grating configuration.”
Case 2. One can use M+N−1 output ports. This is done in Churin and Bayvel, Photonics Technol. Letts. 11(2), 22 (1999). In which 90 inputs are connected to 179 outputs. Also see Churin et al., Electronics Letters 34(12), 1225 (1998). This is undesirable because, for only N wavelengths, certain inputs can never transmit to certain outputs. For the routing table described above, for example, input 2 could not transmit to output 1 (because λN doesn't wrap-around, instead it's routed to output N+1). This is highly undesirable because it reduces system connectivity. It can only be avoided by adding more system wavelengths, as in Case 1 above.
Case 3. Optoelectronic conversion. One can use a wavelength demultiplexer on each port to convert every wavelength into a separate electronic signal, and then use electronic wiring to route these signals to appropriate optical transmitters on different wavelengths, and then use a wavelength multiplexer to combine these signals onto the output fibers. This is done in U.S. Pat. No. 5,742,414, issued Apr. 21, 1998 to N. Frigo et al. The disadvantage is that it adds a lot of cost (the optoelectronic conversion devices, many WDM multiplexer units), and it prevents transparent data transport (the electronic routing will limit the data rate, and possibly data format).
There are also other routing devices which can be configured by cascading many 2×2 (2 input, 2 output) routing elements. These generally are expensive (many components, lots of assembly), and often are restricted to single-mode fiber operation. Examples include: U.S. Pat. No. 5,721,796 issued Feb. 24, 1998 to M. deBarros et al., and U.S. Pat. No. 5,719,976 issued Feb. 17, 1998 to C. H. Henry et al.
The wavelength routing device of the present invention is based on a diffraction grating which utilizes only N wavelengths to interconnect N inputs to N outputs in a fully non-blocking manner. The basic approach is to augment the grating with additional couplers or wavelength selective elements so that N−1 of the 2N−1 outputs are combined with the other N outputs (leaving only N outputs).